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| Home > Publications database > Measurements of Atmospheric OH and HO$_{2}$ Radicals by Laser-Induced Fluorescence on the HALO Aircraft during the OMO-ASIA 2015 Campaign |
| Book/Dissertation / PhD Thesis | FZJ-2020-02698 |
2020
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-477-5
Please use a persistent id in citations: http://hdl.handle.net/2128/25489
Abstract: The goal of this work was to investigate the chemistry of atmospheric OH and HO$_{2}$ radicals in the upper troposphere during the Asian summer monsoon period2015 within the Oxidation Mechanism Observation (OMO) campaign. Concentrations of OH and HO$_{2}$ were measured by a laser-induced fluorescence instrument (AirLIF) on the German research aircraft HALO between the Mediterranean Sea and the Maldives in the Indian Ocean. The measured data are compared to theoretical model predictions in order to test the understanding of atmospheric oxidation processes. For this purpose the precedingly developed AirLIF instrument at Forschungszentrum Jülich was thoroughly characterized in the laboratory and different calibration concepts applied and compared. The radical measurements during OMO were then evaluated and a zero-dimensional chemical box-model calculation for the expected OH and HO$_{2}$ radical concentrations was tested against the measurement results. For the radical measurements using the AirLIF instrument on HALO, the ambient air is first sampled and decelerated by a factor of 10 inside a shrouded inlet. The air is then expanded into a measurement cell at low pressure inside the aircraft, where OH is detected by laser excited fluorescence. The OH and HO$_{2}$ channel of AirLIF needed to be characterized for the flight conditions during OMO. Different calibration concepts have been applied and combined to determine the OH and HO$_{2}$ detection sensitivities as a function of flight altitude, ambient pressure and temperature. These include the well-established ground-based calibrations between flights to track the absolute sensitivities. The relative dependence with altitude was measured in the laboratory using a newly designed photochemical radical source which allows calibration at reduced pressure to simulate ambient air pressure at flight conditions. For the OH-channel as an additional option an in-flight calibration unit inside the shrouded inlet was used. It is however limited to below 10 km, because the radical production by the artificial photolysis of ambient water vapour becomes too small. To simulate the in-flight conditions, other research groups have confided inusing different nozzle sizes to change the mass-flow through the system ins-tead of varying ambient pressure. As part of a consistency check, both methods have been compared in detail and it is confirmed that they essentially agree. However, discontinuities in the pressure dependence of the OH [...]
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